3. OBSERVATIONS OF THE VIRGO CLUSTER

Because of its proximity, the Virgo cluster serves as a prime laboratory
for the
study of environmental influences because of the large number of
observational tools
that can be used to study its member galaxies. Although not a terribly
rich cluster,
the Virgo environment is nonetheless conducive to the
conditions in which we expect
to find morphological alteration and gas sweeping. Therefore, it is
important to review the evidence for gas deficiency in Virgo at this point.

To date, HI observations have been made of several hundred galaxies in
and around
the Virgo cluster covering a wide variety of
morphologies, types, luminosities, masses
and star formation histories. Several authors have examined the
occurrence of HI deficiency within the cluster (e.g.,
Haynes and Giovanelli
1986;
Hoffman et al. 1988).
A substantial number of galaxies covering a wide range of luminosity and
morphological
type are HI deficient by more than a factor of ten. Haynes and
Giovanelli noted that
the zone of HI deficiency extends through the region within about three
degrees of M87.
Within that zone, nearly all galaxies not seen in projection seem to be
effected.

Several studies have used single dish observations to investigate the
relative extent of
the HI and stellar distributions of Virgo galaxies (e.g.,
Helou et al. 1981).
Hewitt et al. (1983)
matched a model HI distribution with Arecibo flat feed major axis
mapping
observations to derive the characteristic HI sizes of large angular
diameter galaxies from the CIG.
Haynes and Giovanelli
(1983)
then compared the HI radii measured
similarly for Virgo spirals with those derived by Hewitt et al. They
found that galaxies
within the five degree Virgo core have HI-to-optical sizes a factor of
two smaller on
average than galaxies further from the cluster center. At the same time,
the HI sizes are
reduced along with the HI masses so that the globally averaged HI
surface density scaled
with the HI (not optical) radius < logMHI /
D2HI > remains constant. In all respects
the peripheral galaxies resembled the general isolated galaxy
sample. Furthermore,
Haynes and Giovanelli
(1986)
emphasized the one-to-one correspondence
between large HI deficiency and small HI-to-optical disk size.

Aperture synthesis studies of the HI emission in galaxies in the Virgo cluster have
been undertaken both with the Westerbork Synthesis Radio Telescope by Warmels
(1988a,
b)
and with the Very Large Array (VLA)
(van Gorkom and
Kotanyi 1985;
Cayatte et al. 1989).
Both studies confirm the earlier single dish
findings that the HI is
preferentially removed from the outer parts of the galaxy and that the
zone of stripped
galaxies extends to about three degrees from M87.
Warmels (1986) has
found not only
a systematic shrinking in the HI diameter relative to the optical for
the subset within
the core, but also concluded that the ratio DHI /
Dopt decreases fairly smoothly with
distance from M87. He identified the region within three to five degrees
as a transition
region where only moderate reduction of DHI / Dopt
is evident. This scale is comparable to the zone of HI deficiency noticed by
Haynes and Giovanelli
(1986).

The VLA surveys add more information about the structure within the HI
distribution and its velocity field. For more discussion of the rotation
curve issue, the
reader is referred to Whitmore's chapter in this volume and to
Guhathakurta et
al. (1988).
It is clearly evident from the global display of the HI
distribution in Virgo
spirals shown by Jaqueline van Gorkom
(van Gorkom and
Kotanyi 1985),
that the inner spirals have shrunken HI disks. Furthermore, the higher
resolution maps presented
by Cayatte et al. (1989)
show peculiar details of the HI distribution. Galaxies in the
western half of the cluster are typical larger in HI than objects in the
eastern part,
and several galaxies show HI asymmetry with a sharp edge on the side of
the galaxy
oriented toward M87 and a more extended distribution on the opposite
side. In Virgo,
we have the opportunity to identify and study galaxies in a current
stripping stage.

The proximity of the Virgo cluster makes it possible to examine the molecular
content, as derived from the millimeter transitions of CO, in galaxies
at different distances
from M87. Two major surveys of CO in Virgo spirals have
been contributed by
Kenney and Young (1986)
and by Stark et
al. (1986).
Galaxies that are HI deficient have
systematically larger ratios of CO flux to HI flux. If the normal
conversion from CO flux
to H2 mass is applied, the core spirals that are deficient by
a factor of ten or more in HI
may be gas-poor (HI + H2) by only a factor of two to
three. Some spatial mapping has
been performed so that an estimate of size of the molecular cloud
distribution relative
to the HI and optical disks can also be made. The spatial distributions
of CO in HI
poor galaxies are not smaller than those of similar objects seen in the
field. It appears
that the peripheral diffuse HI clouds are swept away, but the high
density molecular clouds remain relatively undisturbed throughout the process.

Another estimate of the dust content within galaxies is provided by the
far infrared
emission detected by the Infrared Astronomical Satellite (IRAS),
although the precise
derivation of dust mass is complicated by the mixture of dust components
at different
temperatures and the unknown size distribution of the grains. In order
to relate the dust and gas properties,
Doyon and Joseph (1989)
have found that the HI deficient
galaxies in Virgo have lower 60 and 100 micron fluxes and inferred
temperatures that
are cooler than those with normal HI content. With the caution that
several factors
could play critical roles in the observed emission at the IRAS
wavelength bands, those
authors conclude that at least half of the cool diffuse dust has been
removed from typical
Virgo core spirals. This cool dust, of course is the
cirrus identified with the atomic
hydrogen component of the interstellar medium. Indirectly,
van den Bergh (1984)
has explained the inclination dependence of deviations of individual
galaxies from the mean
blue Tully-Fisher relation in terms of a lowered dust content among
Virgo spirals.

Numerous authors have pointed out peculiarities in the properties of
Virgo galaxies
that can be explained by morphological alteration. Peterson et
al. (1979) have claimed
that the optical disks of early type galaxies in Virgo are smaller than
those in the field.
Bosma (1985)
has examined the disk diameters of Virgo members at the
same surface
brightness limits and finds that although some field spirals have low
surface brightness
extensions, noVirgo galaxies have large outer disks.
Forman et al. (1979)
interpret individual galaxy X-ray sources (e.g., M84, M86, NGC 4388) as ram
pressure sweeping events.

There are numerous other clues as to the star formation rate and history
in Virgo cluster objects that are likely to be of relevance.
Stauffer (1983)
has found a much
higher occurrence of the anemic phenomenon among Virgo spirals than his field sample.
Kennicutt (1983)
notes that the average Virgo spiral of a given morphological
type is redder by about 0.07 in (B - V) than a field object of the same
class. Taken
alternatively, for the same color, a Virgo spiral appears
morphologically about half a
Hubble class later than a field spiral. Kennicutt points out that, in
order to account
for anemia and the color shift, one must both get rid of the blue
galaxies and make all the galaxies redder.
Kennicutt and Kent
(1983)
derive a significantly reduced high
mass star formation rate for Virgo core galaxies from integrated
H measurements.
Indeed, the HI deficiency in Virgo is seen to correlate with the (B - V)
colors, the disk
line emission and the (U(242 1Å) - V) colors in the sense that the HI
poor galaxies are always redder
(Guiderdoni and
Rocca-Volmerange 1985).
Even the dwarf distribution
appears to have been reddened by a reduction of star formation
(Gallagher and Hunter
1986).
Kennicutt (1983)
suggests that the reddening can be explained as a reduction in
the star formation rate by a factor of two about 109 years ago.

The combination of all of this evidence leads us to believe that spirals
that pass
through the center of the Virgo cluster can lose as much as ninety
percent of their HI
mass and suffer a reduction of their star formation rate by about a
factor of two. At
the same time, the molecular constituent remains relatively intact. At
larger distances,
we are not able to study the stripping event in as much detail as in
Virgo, but can use
our knowledge of the Virgo cluster to ask the appropriate questions.